Five men have pleaded not guilty to charges they gang-raped and murdered an Indian trainee physiotherapist, in a case that led to a shake-up of laws against sexual crimes after protests about a rising number of attacks on women.

The men on Saturday entered the court room with their faces covered, and lawyers in the case said they were read 13 charges including murder, which carries a maximum penalty of death. They left after 15 minutes.

"After the judge read out the charges, the five pleaded not guilty and walked out" said A.P. Singh, a lawyer defending two of the accused, Vinay Sharma and Akshay Thakur.

Singh said the next hearing will be held on February 5, when the prosecution will call three witnesses to the formal start of the trial.

A sixth person police say was part of the gang that attacked the woman and her friend is a juvenile and will be tried separately.

Police say the gang lured the 23-year-old physiotherapy student onto a bus, where they repeatedly raped and assaulted her with a metal bar before throwing her bleeding onto a highway. She died of internal injuries two weeks later.

On Friday, India's cabinet approved harsher punishments for rapists, including the death penalty.

The changes, which must be approved by President Pranab Mukherjee to become law, include doubling the minimum sentence for gang-rape and imposing the death penalty when the victim is killed or left in a vegetative state.

"We have taken swift action and hope these steps will make women feel safer in the country," Law Minister Ashwani Kumar told reporters late on Friday.

"This is a progressive piece of legislation and is consistent with the felt sensitivities of the nation in the aftermath of the outrageous gang-rape," he added.

The gang-rape victim's brother on Saturday praised the cabinet's decision to make sentences tougher for attackers, calling it a "positive initiative", according to the Press Trust of India news agency.

The changes to the rape laws were expected to be approved by Mukherjee as early as this weekend but must be ratified by parliament or they will lapse.

Under the changes, the minimum sentence for gang-rape, rape of a minor, rape by policemen or a person in authority will be doubled to 20 years from 10 and can be extended to life without parole.

Under the current law, a rapist faces a term of seven to 10 years, The cabinet has also created a new set of offences such as voyeurism and stalking that will be included in the new law.